Ion Hybrid Capacitors Research Articles

All-Organic Potassium Ion Hybrid Capacitor

A hybrid ion capacitor (HIC), which is a merger of the best properties of a battery and a supercapacitor, has been attracting much interest in recent times. Parallelly, sodium- and potassium-based devices are actively explored beyond lithium chemistry. In an attempt to develop organic material-based energy storage devices, we demonstrate the fabrication of an all-organic potassium ion hybrid capacitor (KIHC). The all-organic hybrid device employs dipotassium terephthalate (K2TP) as the anode and polyaniline (PANi) as the cathode. Along with the cyclic voltammetry method, we use the “three-dimensional Bode analysis” technique to study the charge storage mechanism in these electrodes. The full-cell device exhibits excellent electrochemical properties demonstrating a high energy density of 153 W h kg–1, a high power density of 6795 W kg–1, and good cycle stability up to 10,000 cycles. The results are promising for building all-organic energy storage devices with improved energy and power densities toward building a sustainable and ecofriendly energy storage economy.

Pseudocapacitance-rich carbon nanospheres with graphene protective shield achieving favorable capacity-cyclability combinations of K-ion storage

Pseudocapacitance points out a new direction for the recent research and development of potassium ion anode materials because of its fast energy storage kinetics, which has yielded lots of successful pseudocapacitance-rich examples by introducing more active sites such as doped heteroatoms. However, the negative reactions with the electrolyte in electrochemical process often cause the serious inactivation of active sites, giving a hit for seeking higher pseudocapacitance. In this work we proposed a new strategy to wrap the protective shield for the pseudocapacitance-rich materials to alleviate this matter. By the ultrasonic-induced rGO self-encapsulation of S, N dual-doped carbon spheres, the directional contact between active sites and the electrolyte is blocked, while the rGO films with enlarged interlayer spacing and lots of defects also like filter membrane, accelerate the K+ desolvation and fast diffusion into carbon matrix. Benefiting from the synergistic effect of rGO shielding and filtrating, the pseudocapacitance of S, N dual-doping carbon spheres is fully activated. As the potassium-ion battery anode, the obtained S, N dual-doping carbon sphere@rGO delivers a state-of-the-art capacity of 596 mAh g−1, which achieves a 52.5 % capacity enhancement in contrast to that without rGO shield. As a proof of application, the resultant high-pseudocapacitance carbon anode is incorporated into a potassium ion hybrid capacitor, showing the most favorable capacity-cyclability combinations with the high energy–density of 83 Wh kg−1 after 6500 cycles at 5A g−1.

High energy superstable hybrid capacitor with a self‐regulated Zn/electrolyte interface and 3D graphene‐like carbon cathode

AbstractRechargeable aqueous zinc ion hybrid capacitors (ZIHCs), as an up‐and‐comer aqueous electrochemical energy storage system, endure in their infancy because of the substandard reversibility of Zn anodes, structural deterioration of cathode materials, and narrow electrochemical stability window. Herein, a scalable approach is described that addresses Zn‐anode/electrolyte interface and cathode materials associated deficiencies and boosts the electrochemical properties of ZIHCs. The Zn‐anode/electrolyte interface is self‐regulated by alteration of the traditional Zn2+ electrolyte with Na‐based supporting salt without surrendering the cost, safety, and green features of the Zn‐based system which further validates the excellent reversibility over 1100 h with suppressed hydrogen evolution. The deficits of cathode materials were overcome by using a high‐mass loaded, oxygen‐rich, 3D, multiscaled graphene‐like carbon (3D MGC) cathode. Due to the multiscaled texture, high electronic conductivity, and oxygen‐rich functional groups of 3D MGC, reversible redox capacitance was obtained with a traditional adsorption/desorption mechanism. Prototype ZIHCs containing the modified electrolyte and an oxygen‐rich 3D MGC cathode resulted in battery‐like specific energy (203 Wh kg−1 at 1.6 A g−1) and supercapacitor‐type power capability (4.9 kW kg−1 at 8 A g−1) with outstanding cycling durability (96.75% retention over 30 000 cycles at 10 A g−1). These findings pave the way toward the utilization of highly efficient ZIHCs for practical applications.image

N, P dual doped foamy-like carbons with abundant defect sites for zinc ion hybrid capacitors

To achieve high value-added use of biomass waste is still a big challenge. Besides, heteroatom-doped carbon materials have been charmed enormous interests due to their high conductivity and abundant defect sites. Herein, a simply and confined pyrolysis method is innovated to prepare N, P codoped foamy-like carbons (NPFC) with abundant defect sites from osmanthus flowers. The NPFC features uniform N, P doped, porous structures and big electric conductivity, which provide abundant defects and additional active sites for ion adsorption, shorten ion transfer distance, enhance transfer kinetics and supply highways for electron transfer. As cathode for zinc ion hybrid capacitor (ZHC), NPFC exhibits large energy density (85.7 Wh kg−1) with long cycle life (remains 97.4% after 20,000 cycles) in aqueous electrolyte. More interestingly, NPFC-based quasi-solid-state ZHC presents extremely big capacity of 163.6 mAh g−1 at 0.1 A g−1 and excellent power density of 35.9 kW kg−1. This study offers a universal method to synthesize heteroatom-doped carbon materials from biomass waste for hybrid capacitors.

All 3D Printing Shape‐Conformable Zinc Ion Hybrid Capacitors with Ultrahigh Areal Capacitance and Improved Cycle Life (Adv. Energy Mater. 27/2022)

Zinc-Ion Hybrid Capacitors In article number 2200341, Shuanghao Zheng, Zhong-Shuai Wu and co-workers demonstrate a new strategy for the fabrication of shape-conformable and all-3D-printed zinc-ion hybrid capacitors, including hierarchically porous micro-lattice cathodes, metal anode stabilizers, quasi-solid gel electrolytes and plastic packages. This strategy provides a general and innovative methodology for constructing custom-shape electrochemical energy storage devices with satisfactory electrochemical performance.

Fast ion diffusion alloy layer facilitating 3D mesh substrate for dendrite-free zinc-ion hybrid capacitors

Although aqueous zinc ion hybrid capacitors have advantageous integration of batteries and supercapacitors, they still suffer from the inherent problems of dendrite growth and interfacial side reactions on Zn anodes. Herein, a universal fast zinc-ion diffusion layer on a three-dimensional (3D) mesh structure model is demonstrated to effectively improve Zn plating/stripping reversibility. The fast ion diffusion alloy layer accelerates the Zn2+ migration in an orderly manner to homogenize Zn2+ flux and overcomes the defects of the commercial mesh substrate, effectively avoiding dendrite growth and side reactions. Consequently, the proof-of-concept silver-zinc alloy modified stainless steel mesh delivers superb reversibility with the high coulombic efficiency over 99.4% at 4 mA cm−2 after 1600 cycles and excellent reliability of over 830 h at 1 mA cm−2, Its feasibility is also evidenced in commercial zinc ion hybrid capacitors with activated carbon as the cathode. This work enriches the fundamental comprehension of fast zinc-ion diffusion layer combined with a 3D substrate on the Zn deposition and opens a universal approach to design advanced host for Zn electrodes in zinc ion hybrid capacitors.

G-C3N4 templated mesoporous carbon with abundant heteroatoms as high-rate anode material for dual-carbon sodium ion hybrid capacitors

Sodium ion hybrid capacitors (SIHCs) are regarded as advanced power supply systems. Nevertheless, the kinetics imbalance of cathode and anode suppresses the further performance improvement of SIHCs. The carbonaceous anode materials are promising and many strategies have been utilized to increase the capacity of sloping region or accelerate the reaction rate of plateau region. However, it is still challenging to simultaneously realize high mesopore/micropore volume ratio, large interlayer distance (>0.37 nm), and abundant and favorable heteroatoms-doping by a simple method. Herein, we report N, P, O ternary-doped mesoporous carbon (PNPOC-T, T = 700, 800 or 900) with large interlayer distance (∼0.4 nm) as anode materials. The PNPOC-T were prepared by a simple in-situ polymerization of aniline and phytic acid on the exfoliated graphitic nitrogen carbide (g-C3N4) and subsequent carbonization. The obtained PNPOC-800 exhibits an excellent rate performance (101.5 mA·h ·g−1 at 20 A·g−1), which can be attributed to the high surface-controlled capacitive behavior ratio and rapid ion diffusion. The optimum SIHCs display a high energy density of 105.48 W·h·kg−1 and a high power density of 13.59 kW·kg−1. Furthermore, the capacitance retention rate of SIHCs can reach 87.43% after 9 000 cycles at 1 A·g−1.

Rational designed hierarchical dual carbon protected ZnSe anode for advanced sodium-ion hybrid capacitors

With a remarkable advantage of high theoretical capacity and excellent rate performance, zinc selenide (ZnSe) has been regarded as a high-potential electrode for sodium-ion batteries (SIBs). Nevertheless, the practical use of ZnSe as the anode for SIBs is severely limited by the large volumetric expansion and sluggish kinetics during the conversion/alloying reaction process. Herein, we develop a dual-type carbon approach to afford a 3D hierarchical ZnSe@N-doped carbon/reduced graphene oxide (ZnSe@NC/rGO) using simple selenization of ZIF-8 and subsequent incorporation process. Alongside with buffering the volume variation by the in-situ generated NC, the electron and ion transfer can also be accelerated on account of the interconnected graphene network. Therefore, the ZnSe@NC/rGO offers a splendid discharge capacity of 455.3 mAh g−1 at 0.1 A g−1, and exhibits a superior rate capability of 195.1 mAh g−1 at 5 A g−1. Besides, the capacity retains as high as 170.1 mAh g−1 after 500 cycles at 5 A g−1, indicating an outstanding cyclic ability. A full-cell sodium ion hybrid capacitor (SIHC) constructed using a ZnSe@NC/rGO anode and a commercial activated carbon (AC) cathode (ZnSe@NC/rGO//AC SIHC) shows a high energy output of 117.8 Wh kg−1 at 105 W kg−1, and a long lifespan with 86.4% capacity retention over 3000 cycles.

N, S co-doped porous carbons with well-developed pores for supercapacitor and zinc ion hybrid capacitor

The introduction of heteroatoms into carbon materials is beneficial for enhancing zinc ion storage capability, and thereby obtaining excellent electrochemical performance. Herein, the N, S co-doped porous carbons (NSPCs) with well-developed pores and massive surface area are synthesized from natural biomass by the synergistic activation strategy of KHCO 3 and Na 2 S 2 O 3 . Accordingly, NSPC K achieves high capacitance (262.7 F g −1 ) and long cycle life (remains 99.3% after 10,000 cycles) in 1 M Zn(CF 3 SO 3 ) 2 aqueous electrolyte for supercapacitor. Impressively, the NSPC cathode presents high discharge capacity of 136.3 mAh g −1 in the voltage window of 0–1.8 V, good rate performance (86.5 mA h g −1 at 20 A g −1 ) and high energy density (122.6 W h kg −1 ) for zinc ion hybrid capacitor (ZHC). Besides, the ZHC delivers long-term life with 0.5% damage after 15,000 cycles. This study may facilitate the design of high-energy and safe electrochemical energy storage devices based on the heteroatom doping carbons and aqueous electrolyte. The N, S co-doped porous carbons with well-developed pores and massive surface area are synthesized from natural biomass by the synergistic activation strategy of KHCO 3 and Na 2 S 2 O 3 , displaying high specific capacity and energy density in aqueous two-electrode system. • The N, S co-doping porous carbon (NSPC K ) was prepared by confined pyrolysis method. • The NSPC K presents well-developed pore structures and has a moderate content of N, S. • The NSPC K shows high specific capacity in 1 M Zn(CF 3 SO 3 ) 2 electrolyte. • The NSPC K cathode displays high energy density and superb cycle stability.

All 3D Printing Shape‐Conformable Zinc Ion Hybrid Capacitors with Ultrahigh Areal Capacitance and Improved Cycle Life

AbstractAqueous zinc ion hybrid capacitors (ZIHCs) with notable advantages like cost‐effectiveness and safety are promising new‐generation electrochemical energy storage devices (EESDs). However, unsatisfactory areal capacitance, zinc dendrites, and rigid form factor of typical aqueous ZIHCs impede their further practical applications. Herein, a protocol is developed for 3D‐printing shape‐conformable ZIHCs, including hierarchically porous microlattice cathode, metal anode stabilizer, quasisolid gel electrolyte, and plastic packages, through direct ink writing and fused deposition modeling methods. Among these components, microlattice cathodes with a high mass loading density of 22.8 mg cm−2 effectively improve the areal capacitance of ZIHCs, and the metal anode stabilizer sufficiently inhibits dendritic growth to prolong the cycle life of ZIHCs. As a result, the obtained all 3D‐printed ZIHCs deliver a high areal capacitance of 4259 mF cm−2 as well as a considerable energy density of 1514 μWh cm−2, surpassing most reported ZIHCs. The 3D‐printed metal anode stabilizer endows ZIHCs with better cycling stability with almost no capacitance loss after 10 000 cycles. Impressively, the ZIHCs can easily be manufactured into shape‐conformable devices encapsulated by 3D‐printed polylactic acid package, demonstrating their viability. Therefore, this all 3D‐printing strategy provides a general and innovative methodology for constructing customized EESDs.

Highly Transparent and Flexible Zn-Ti3C2Tx MXene Hybrid Capacitors.

With the development of transparent and wearable electronic devices, energy supply units with high transmittance and flexibility, long cycle life, and high power and energy density are urgently needed. Zinc ion hybrid capacitors (ZIHCs) combined with the advantages of both supercapacitors and zinc ion batteries are promising energy supply components in the abovementioned devices. In addition, the preparation of multifunctional devices has become a trend for the need of space- and resource-saving. Therefore, obtaining ZIHCs with high transmittance and exploring their potential applications are meaningful challenges. Herein, a transparent and flexible ZIHC composed of a patterned zinc foil anode, transparent MXene cathode, and ZnSO4-polyacrylamide (PAM) hydrogel electrolyte is designed and realized. The ZIHC exhibits a superior capacitance of 318 μF cm-2 (5 mV s-1) with 94% transmittance and retains 76% of the initial capacitance after 10,000 charge-discharge cycles. It also shows excellent flexibility, i.e., its capacitance has no obvious attenuation under different bending states. Interestingly, the leakage current of the ZIHC is highly sensitive to electric fields, which shows potential application in electric field detection. This work presents a method to realize the multifunctional ZIHC with electric field sensing function for transparent and flexible wearable devices in the future.

Bead‐Like Coal‐Derived Carbon Anodes for High Performance Potassium‐Ion Hybrid Capacitors

AbstractThe pursuit of long life, high rate and low‐cost anode materials is of great significance for potassium‐ion hybrid capacitors (PIHCs). Here, the bead‐like coal‐derived carbon films (BCCFs‐800) for PIHCs were synthesized with low‐cost coal as precursor. Benefiting from the well‐designed composite structure, the wettability on the electrode surface was enhanced and the diffusion of electrolyte ions was also facilitated, thereby improving the electrochemical performance of the electrode. A potassium ion hybrid capacitor was fabricated with BCCFs‐800 as battery‐type anode and active carbon as capacitor‐type cathode, and it delivered promising energy and power densities (119 Wh kg−1 at 186 W kg−1 and 2187 W kg−1 at 52 Wh kg−1). The as‐obtained earth‐abundant and high performance electrodes will promote the green and clean utilization of coal and accelerate the development of potassium ion electrochemical storage technology.

A Review on the Conventional Capacitors, Supercapacitors, and Emerging Hybrid Ion Capacitors: Past, Present, and Future

Electrochemical energy storage (EES) devices with high‐power density such as capacitors, supercapacitors, and hybrid ion capacitors arouse intensive research passion. Recently, there are many review articles reporting the materials and structural design of the electrode and electrolyte for supercapacitors and hybrid capacitors (HCs), though these reviews always focus on individual supercapacitors or single HCs. Herein, the conventional capacitor, supercapacitor, and hybrid ion capacitor are incorporated, as the detailed description of conventional capacitors is very fundamental and necessary for the better understanding and development of supercapacitors and hybrid ion capacitors, which are often ignored. Therefore, herein, the fundamentals and recent advances of conventional capacitors, supercapacitors, and emerging hybrid ion capacitors are comprehensively and systematically summarized in terms of history, mechanisms, electrode materials, existing challenges, and perspectives. At the same time, it is believed that a comprehensive and fundamental understanding for capacitor‐related EES devices is provided in the review and has a great guiding role for future development.

Homologous Strategy: Fabrication of a High-Performance Dual-Carbon Potassium Ion Hybrid Capacitor

Potassium ion hybrid capacitors (PIHC) have attracted a significant amount of interest in recent years because of their excellent overall performance and the abundance of potassium resources. The search for kinetically matched and high-performing anode and cathode materials is the key to the development of PIHCs. Hereby, we present a homologous preparation method for dual-carbon electrode materials of PIHCs with the single precursor nitrilotriacetic acid trisodium salt (NTA-3Na) by different preparation routes. The 3NaC-700 obtained by high-temperature calcination as the anode exhibits excellent kinetics performance for potassium ion storage due to its three-dimensional, large layer spacing and N-doped structure. 3NaC can be converted to a 3NaAC cathode with a high specific surface area (SSA, 3014 m2 g–1) and superior capacitance properties by further KOH activation. As a result of the advantages of the 3NaC and 3NaAC electrodes mentioned above, a high-performance dual-carbon PIHC is fabricated. It delivers a high energy and power density of 149.2 Wh kg–1 at 200 W kg–1. A high energy density of 67.2 Wh kg–1 is maintained even at a high power density of 20 kW kg–1. The PIHC device also exhibits excellent cycling stability with 81% capacity retention after 8000 cycles at a current density of 2.0 A g–1.

Enabling Multi-Chemisorption Sites on Carbon Nanofibers Cathodes by an In-situ Exfoliation Strategy for High-Performance Zn–Ion Hybrid Capacitors

HighlightsThe high pyridine/pyrrole nitrogen-doped and carbonyl-functionalized nanosheets on flexible electrospun porous carbon nanofibers film cathode integrates the advantages of large specific surface area, strong hydrophilicity and high Zn2+ adsorption activity.The Zinc-ion hybrid capacitors exhibits a high capacitance retention of 99.2% over 200,000 cycles at 40 A g−1.A synergistic Zn2+ storage mechanism between carbonyl and pyridine/pyrrole nitrogen atoms is proposed.

VS4 nanoarrays pillared Ti3C2Tx with enlarged interlayer spacing as anode for advanced lithium/sodium ion battery and hybrid capacitor

Aiming to improve the sluggish ion insertion/extraction at the battery-type anode in lithium-ion/sodium-ion hybrid capacitor (LIC/SIC), herein, a VS4@L-Ti3C2Tx composing of VS4 nanoarrays and MXene with enlarged interlayer spacing is developed through a facile hydrothermal approach. Benefiting from the merits of building blocks, the VS4@L-Ti3C2Tx based lithium-ion half-cell exhibits superior specific capacity (1328 mAh g−1 after 170 cycles at 0.1 A g−1), rate performance (808 mAh g−1 after 10 cycles at 20 A g−1) and cycling stability (721 mAh g−1 after 500 cycles at 1.0 A g−1), surpassing most of the reported VS4 based counterparts. The sodium-ion half-cell with VS4@L-Ti3C2Tx anode also shows good performances. Moreover, LIC and SIC based on VS4@L-Ti3C2Tx anodes deliver high energy densities of 122.5 Wh kg−1 and 117.0 Wh kg−1 at a low power density 100.3 W kg−1, respectively. Additionally, they exhibit excellent cycling lifespans with capacity retention of 60% after 20000 cycles for LIC at a high current rate 1.0 A g−1, outperforming most of the reported analogues. This study may shed light on the development of other layered anode materials for energy storage devices.

Engineering Electronic Transfer Dynamics and Ion Adsorption Capability in Dual-Doped Carbon for High-Energy Potassium Ion Hybrid Capacitors

Sodium and potassium ions energy storage systems with low cost and high energy/power densities have recently drawn increasing interest as promising candidates for grid-level applications, while the lack of suitable anode materials with fast ion diffusion kinetics highly hinders their development. Herein, we develop a nanoscale confined in situ oxidation polymerization process followed by a conventional carbonization treatment to generate phosphorus and nitrogen dual-doped hollow carbon spheres (PNHCS), which can realize superior sodium and potassium ion storage performance. Importantly, the density functional theory calculation and combined characterizations, e.g., in situ Raman spectroscopy and ex situ X-ray photoelectron spectroscopy, decipher that the P/N doping can enhance the electronic transfer dynamics and ion adsorption capability, which are responsible for enhanced electrochemical performance. Inspiringly, the practicability of the PNHCS anode is demonstrated by assembling the potassium ion hybrid capacitors (KIHCs), where the prominent energy density is 178.80 Wh kg-1 at a power density of 197.65 W kg-1, with excellent cycling stability, can be achieved. This work not only promotes the development of efficient anode material for sodium/potassium ion storage devices but also deciphers the embedded ion storage mechanism.

2D molten salt strategy for preparing large-sized MoS2/C sheets with self-adaptive structural deformation for K-ion storage

Due to the unique two-dimensional (2D) structure and large interlayer distance, MoS2 is considered as one potential anode material for fast-developed potassium ion energy storage. Differing to the intensive study on nano-sized MoS2, in this work our attention is focused on the probability of large-sized MoS2 sheets. Following this, a micrometer-scale crystalline MoS2 sheet with few-stacked layers and enlarged interlayer spacing anchored on carrageenan-derived carbon is fabricated by a novel 2D molten salt approach. Controlled by the 2D space-confined molten salt layers on the surface of KCl crystals, the oriented growth of MoS2 flakes creates the few layer-stacking MoS2 sheets in microscale. Meantime, the insertion of K+ in molten salts enlarges the interlayer distance and induces the production of 1 T phase. Besides, a novel self-adaptive structural deformation from MoS2/C sheets to bubble-wrap-like architecture has been discovered during electrochemical process, demonstrating the specific way of large-sized MoS2 sheets to alleviate volume expansion and exfoliation resulted from K-ion intercalation. Based on above synergistic effect, the optional samples exhibit a high capacity of 413 mAh g−1 at 0.1 A g−1, and an excellent cycling capability with a retention of 89.7% after 2000 cycles at 2 A g−1. The assembled potassium ion hybrid capacitor delivers a high energy of 130 Wh kg−1 and superior reversibility with 85.1% retention after 10,000 cycles.

Urea-Mediated Monoliths Made of Nitrogen-Enriched Mesoporous Carbon Nanosheets for High-Performance Aqueous Zinc Ion Hybrid Capacitors.

Aqueous zinc ion hybrid capacitors (aZHCs) are of great potential for large-scale energy storage and flexible wearable devices, of which the specific capacity and energy density need to be further enhanced for practical applications. Herein, a urea-mediated foaming strategy is reported for the efficient synthesis of monoliths consisting of nitrogen-enriched mesoporous carbon nanosheets (NPCNs) by prefoaming drying a solution made of polyvinylpyrrolidone, zinc nitrate, and urea at low temperatures, foaming and annealing at high temperatures, and subsequent acid etching. NPCNs have a large lateral size of ≈40µm, thin thickness of ≈55nm, abundant micropores and mesopores (≈3.8nm), and a high N-doping value of 9.7 at.%. The NPCNs as the cathode in aZHCs provide abundant zinc storage sites involving both physical and chemical adsorption/desorption of Zn2+ ions, and deliver high specific capacities of 262 and 115 mAh g-1 at 0.2 and 10 A g-1 , and a remarkable areal capacity of ≈0.5 mAh cm-2 with a mass loading of 5.3mg cm-2 , outperforming most carbon cathodes reported thus far. Moreover, safe and flexible NPCNs based quasi-solid-state devices are fabricated, which can withstand drilling and mechanical bending, suggesting their potential applications in wearable devices.

3D porous H-Ti3C2Tx films as free-standing electrodes for zinc ion hybrid capacitors

Flexible zinc ion hybrid capacitor (ZICs) as a kind of emerging energy storage devices have great prospect for grid-scale energy storage and wearable electronics due to some advantages such as appropriate density, superb power output, abundant zinc resources, and nontoxic nature. Currently, there are still great demands to seek and design novel electrodes for zinc ion hybrid capacitors, especially for flexible free-standing electrode. Herein, a novel rechargeable aqueous Zn-ion hybrid capacitor using 3D porous H-MXene film (3D-PHMF) as the cathode was reported. The 3D porous H-MXene film was constructed with an interlayer interaction engineering via freeze-casting method, wherein the hydrogen ion (H+) was introduced to weaken electrostatic repulsion of MXene interlayer. The aqueous Zn//3D-PHMF capacitor delivers a specific capacitance of 105.6 mAh g−1 at 0.2 A g−1 and good rate performance of 61.0 mAh g−1 at 5 A g−1 with remarkable cycling stability, which remains 90% of specific capacitance after 20,000 cycles. Besides, the capacitor also shows outstanding anti-self-discharge ability in which the self-discharge rate is 1.87 mV h−1. Remarkably, the as-obtained flexible cathode can be designed into a flexible quasi-solid-state device, which shows excellent flexibility and stability. This work demonstrates the promising possibility of MXene as free-standing electrode for energy storage applications.